1. Field of the Invention
The present invention relates to processes for making aluminum sheet and sheets made thereby, and more particularly relates to processes for making high strength, aluminum sheets including magnesium containing aluminum sheets and sheets made thereby involving batch annealing.
2. Description of Related Art
In conventional manufacturing processes for obtaining aluminum sheet, there is generally a trade off between the ultimate tensile strength, magnesium content and elongation of a material. For example increasing the cold work of the sheet generally results in increased ultimate tensile strength with a decrease in elongation. And as a further example, decreasing the cold work generally results in a decrease in ultimate tensile strength with a corresponding increase in elongation. Increasing magnesium content can improve the ultimate tensile strength of the alloy, but typically causes a corresponding increase in the cost of the alloy.
As set out below, flash annealing processes have been developed to address some of the problems that have heretofore been associated with batch annealing, but flash annealing typically requires an additional handling step of unwinding and re-winding, whereas batch annealing does not require this additional handling.
In contrast to the batch annealing process of the present invention, various flash annealing processes, also referred to as continuous annealing processes, have existed and require the additional step of continuously passing the sheet through a heating means as a single web to provide a heat up rate of the sheet at a greatly increased rate over that of batch annealing.
Examples of such flash annealing processes include Palmer et al. U.S. Pat. No. 5,362,341, issued Nov. 8, 1994, which is incorporated herein by reference; and additional continuous annealing processes are disclosed in Tanaka et al. U.S. Pat. No. 5,062,901, issued Nov. 5, 1991; Tanaka et al. U.S. Pat. No. 5,240,522, issued Aug. 31, 1993; Tanaka et al. U.S. Pat. No. 4,968,356, issued Nov. 6, 1990; Wyatt-Mair et al. U.S. Pat. No. 5,470,405 issued Nov. 28, 1995; Wyatt-Mair et al. U.S. Pat. No. 5,496,423 issued Mar. 5, 1996; Wyatt-Mair et al. U.S. Pat. No. 5,514,228 issued May 7, 1996; Tahara et al. U.S. Pat. No. 5,512,111 issued Apr. 30, 1996; Shoiji et al. U.S. Pat. No. 5,518,558 issued May 21, 1996. Satou et al. U.S. Pat. No. 5,578,114 issued Nov. 26, 1996 involves a continuous casting process; Sanford et al. U.S. Pat. No. 5,547,524 issued Aug. 20, 1996 discloses a process for producing a structurally hardened plate involving heating opposite edges at various temperatures; Gen et al. U.S. Pat. No. 5,616,189 issued April, 1997 discloses a process involving flash annealing; Bekki, et al. U.S. Pat. No. 5,605,586 discloses a process involving flash annealing; Kamat U.S. Pat. No. 5,634,991 issued Jun. 3, 1997 discloses a process involving annealing at the rate of heat up at 75 degrees per hour; all of which are incorporated herein by reference in their entireties.
The various flash annealing processes have typically required the additional step of unwinding and re-winding the coil of aluminum sheet. This winding is both time consuming and costly and the aluminum sheet can be damaged in the process, all of which add to the cost of the product. Batch annealing does not require that the aluminum sheet be unwound, and is thus very desirable. However, conventional batch processing has not obtained the desired combination of high ultimate tensile strength for a given level of elongation and magnesium level. For example, strength levels exceeding 448 MPa (65,000 psi) are currently not available in Alxe2x80x94Mg sheet products, as partially shown in the table provided below in the detailed description.
Consequently there is a need and a desire to provide a batch annealing process in the production of aluminum sheet which will provide a high ultimate tensile strength for a given level of elongation and magnesium level. There is also a desire to increase the ultimate tensile strength for a given magnesium level and the elongation percent in order to permit a reduced gauge thickness of sheet made by the present process to effectively perform as a relatively larger conventional gauge thickness of sheet made by a conventional batch annealing process.
The present invention involves aluminum sheets and methods for manufacturing an aluminum sheet. The present invention further involves control of processing conditions in order to achieve a fine grain size (i.e. ASTM rating of 8.5 or greater) in an aluminum sheet prior to a final cold working operation. Also included within the scope of the present invention are products having a fine grain size which have strength levels above what can be obtained in 5xc3x97xc3x97xc3x97 alloys.
Additional objects, features and advantages of the invention will be set forth in the description which follows, and in part, will be obvious from the description, or may be learned by practice of the invention. The objects, features and advantages of the invention may be realized and obtained by means of the instrumentalities and combination particularly pointed out in the appended claims.